Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. They were first reported in 1991 by Sumio Iijima who produced multi-layer concentric tubes or multi-walled carbon nanotubes by evaporating carbon in an arc discharge. They reported carbon nanotubes having up to seven walls. In 1993, Iijima's group and an IBM team headed by Donald Bethune independently discovered that a single-wall nanotube could be made by vaporizing carbon together with a transition metal such as iron or cobalt in an arc generator (see Iijima et al. Nature 363:603 (1993); Bethune et al., Nature 363: 605 (1993) and U.S. Pat. No. 5,424,054). The original syntheses produced low yields of non-uniform nanotubes mixed with large amounts of soot and metal particles.
Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Journet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al. Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274: 1701 (1996)). Multi-walled carbon nanotubes can be produced on a commercial scale by catalytic hydrocarbon cracking while single-walled carbon nanotubes are still produced on a gram scale.
Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes because they have unique mechanical and electronic properties. Defects are less likely to occur in single-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Defect-free single-walled nanotubes are expected to have remarkable mechanical, electronic and magnetic properties that could be tunable by varying the diameter, number of concentric shells, and chirality of the tube.
It is generally recognized that smaller catalyst particles of less than 3 nm are preferred for the growth of smaller diameter carbon nanotubes. However, the smaller catalyst particles easily aggregate at the higher temperatures required for the synthesis of carbon nanotubes. U.S. Patent Application No. 2004/0005269 to Huang et al. discloses a mixture of catalysts containing at least one element from Fe, Co, and Ni, and at least one supporting element from the lanthanides. The lanthanides are said to decrease the melting point of the catalyst by forming alloys so that the carbon nanostructures can be grown at lower temperatures.
Aside from the size of the catalyst, the temperature of the reaction chamber can also be important for the growth of carbon nanotubes. U.S. Pat. No. 6,764,874 to Zhang et al. discloses a method of preparing nanotubes by melting aluminum to form an alumina support and melting a thin nickel film to form nickel nanoparticles on the alumina support. The catalyst is then used in a reaction chamber at less than 850° C. U.S. Pat. No. 6,401,526, and U.S. Patent Application Publication No. 2002/00178846, both to Dai et al., disclose a method of forming nanotubes for atomic force microscopy. A portion of the support structure is coated with a liquid phase precursor material that contains a metal-containing salt and a long-chain molecular compound dissolved in a solvent. The carbon nanotubes are made at a temperature of 850° C.
It is well known that the diameter of the SWNTs produced is proportional to the size of the catalyst particle. In order to synthesize nanotubes of small diameter, it is necessary to have catalyst particles of small particle size (less than about 1 nm). Catalysts of small average particle sizes with narrow distribution are difficult to synthesize. Further, recognized methods for determining the catalyst particle size distribution are not currently available, especially when the catalyst particles are supported on support powders, and thus buried inside the pores of the support powders.
Thus, there is a need for methods and processes for controllable synthesis of carbon single walled nanotubes with small and narrow distributed diameters. Accordingly, the present invention provides novel methods and processes for determining the average particle size and particle size distribution of catalyst particles that can be used for preparation and optimization of catalyst and for the synthesis of SWNTs with small and narrow distributed diameters.